Programmable logic on integrated circuits was introduced and popularized during the 1970's. Programmable array logic (PAL) and programmable logic devices (PLD) utilized advanced semiconductor processing technology, and enabled customers to purchase standard off-the shelf circuits that were essentially blank, and customize the circuits at the customer location. Unfortunately, the small density of these circuits limited the size and complexity of the designs.
Programmable elements such as fuses are well known for use in semiconductor devices, such as PLDs. See, for example, "Advanced Single Poly BiCMOS Technology for High Performance Programmable TTL/ECL Applications" by Iranmanesh, et al. IEEE 1990 Bipolar Circuits and Technology Meeting. Semiconductor fuse materials include Ti--W and Pt--Si, (see U.S. Pat. No. 4,796,075).
In addition to PALs, PLDs and fuses, Application Specific Integrated Circuits (ASICs) can be used to implement custom logic. The ASICs market exploded during the 1980's with the popularization of the masked gate array. A gate array employed a standard base array which could be stored in inventory and then metallized in accordance with the needs of a particular customer to form a desired logic circuit. Gate array technology enabled complex functions to be designed and delivered within weeks. The masked gate array however, did not give the system designers the flexibility and time-to-market advantage of the PLDs and PALs which were field programmable.
This limitation of ASICs fostered the birth of the Field Programmable Gate Arrays (FPGAs) in the early to mid 1980's. Integrated circuit programmable logic has been built using programmable elements such as, for example, SRAMs (static random access memory), EPROMs (electrically programmable read only memory), fuses and antifuses. Antifuse materials typically used in the semiconductor field include silicon oxide/silicon nitride composites (see, for example, U.S. Pat. Nos. 4,823,181, 4,876,220 and 5,258,643), and amorphous silicon (see, for example, U.S. Pat. Nos. 4,914,055 and 5,196,724). Properties and structures of such antifuse materials are described in the prior art (see, for example, "Dielectric Based Antifuse for Logic and Memory ICs" by Hamdy, et. al., 1988 International Electronic Devices Meeting (IEDM); "Antifuse Structure Comparison for Field Programmable Gate Arrays" by Chiang, et. al. 1992 IEDM; "Interconnect Devices for Field Programmable Gate Array" by Hu, 1992 IEDM; "Conducting Filament of the Programmed Metal Electrode Amorphous Silicon Antifuse" by Gordon, et. al. 1993 IEDM).
Several attempts have also been made to build programmable printed circuit boards (PCBs) and multichip modules (MCMs) as described in U.S. Pat. Nos. 5,055,973, 5,077,451, 4,458,297 and 4,847,792. Programmable printed circuit boards can be built utilizing integrated circuits (silicon chips made using semiconductor technology) at key locations throughout a PCB for switching signals into various programmable paths on the printed circuit board (PCB) as described in U.S. Pat. Nos. 5,055,973, 5,077,451 and Aptix Data Book (Feb. 1993) available from Aptix Corporation, 2880 N. First Street, San Jose, Calif. 95134. The wiring paths in the PCB are directed to and from these programmable silicon chips. However, silicon switches are highly resistive in comparison to circuit substrates and are expensive, adding to the cost of a PCB.
In U.S. Pat. Nos. 4,458,297 and 4,847,792, a silicon circuit board (SCB) with programmable switches was provided and other silicon devices were mounted on top of the SCB. Eventually the SCB was packaged and resulted in a programmable multichip module (MCM) package. The silicon circuit board as well as switches were made using semiconductor IC technology.
For ceramic packages, methods of making engineering change contact pads on a top surface have been described in U.S. Pat. No. 4,840,924 to Kinbara. However, such prior art contact pads are provided on a top surface layer, are cut by mechanical or laser cutting and have connection conductor portions to connect a wire for making the wiring change.
The approaches mentioned above for programmable PCBs, SCBs, MCMs and ceramic packages have several drawbacks. For example, their time-to-market is slow and their cost is high. Therefore a new approach is necessary.